1. Field of the Invention
The present invention relates generally to facsimile apparatus, and, more particularly, to a facsimile apparatus which can be operated by DTMF (dual tone multifrequency) signals from an external telephone.
2. Description of the Related Art
Facsimile apparatus or facsimile machines combine copying with transmission. They routinely send and receive facsimile signals over the telephone network, increasing the uses of the telephone systems besides sending and receiving voice signals of the conversations by ordinary telephone sets. The amount of information that can be handled by facsimile apparatus is huge. Therefore, facsimile apparatus have enjoyed tremendous acceptance and growth in homes and businesses.
An external telephone is usually arranged to be connected to the facsimile apparatus so that either one is available on the common telephone line. When the external telephone uses the telephone line for conversation, the facsimile apparatus is sometimes wanted to be connected to the telephone line for the purpose of sending or receiving facsimile images. At that time, operation of switching over to the facsimile apparatus from the external telephone is needed.
Now, there is a type of facsimile apparatus that is remotely operated by the combinations of DTMF signals (numbers 0 through 9 and symbols # and *) sent from the external telephone which is the Touch-Tone telephone having a push-button keypad. That is, the tone-pair signal frequencies sent from the external telephone are tested in the facsimile apparatus mentioned above to determine if they are DTMF frequencies. Then, the tone durations is measured to determine if the tone-pair signal is present at least for a predetermined detection time. The tone-pair signal that accomplishes the timing requirement is recognized as a valid DTMF signal used for the remote operation. The combination of valid DTMF signals may indicate the type of remote operations.
In the facsimile apparatus mentioned above, the remote operation by DTMF signals is performed after the signals have accomplished filtering and timing requirements. Therefore, the remote operation by talk-off, that is the detection of speech segments or other sounds as a real DTMF signal, is effectively prevented.
However, since the tone duration of the DTMF signal is tested on the basis of a predetermined fixed detection time, the facsimile apparatus has the following disadvantages.
That is, as for the electronic telephones which can send DTMF signals, there are provided some models having operating parameters wherein tone durations of DTMF signals generated are 50 ms. 80 ms and 120 ms varying according to the models. And there is still a model in which a DTMF signal is generated while a key of the push-button keypad is pressed. Therefore, the facsimile apparatus having the electronic telephone used as the external telephone may receive DTMF signals having a different tone duration peculiar to the model.
At the beginning of the connection of an external telephone to the facsimile apparatus, it is needed to set the detection time at the minimum time, that is 50 ms (actually a little bit shorter time, say 45 ms), as the detection time, so that every electronic telephone is available as the external telephone for the purpose of remote operations. However, when a external telephone having operating parameters wherein the tone duration of DTMF signals is for example 120 ms, is connected to the facsimile apparatus, there exists a higher probability that the DTMF signal is erroneously recognized compared to the case where the detection time is set at 120 ms (actually a little bit shorter time). Therefore, at the time of erroneous recognision of the signal and erroneous operation in the known facsimile apparatus, it is necessary to reset the dectection time in the range where the facsimile apparatus is allowed to be remotely operated and in the optimum condition adapted to the operating parameters of the external telephone.
As mentioned above, the known facsimile apparatus are arranged to recognize DTMF signals in light of the frequencies and the detection time for preventing the erroneous identification of the DTMF signal. However, the facsimile has disadvantages in that it is inconvenient for the user to manually set the detection time that is optimized to the tone duration of DTMF signals as one of the operating parameters of the external telephone.